In conventional papermaking, a calendering process is carried out in order to improve the smoothness of the surface of the paper being produced. There are various types of calendering apparatus. Typical calendering apparatuses include the machine calender, in which the nip is composed of a pair of steel rolls, and the super calender in which the nip is composed of a steel roll and an elastic roll, the steel roll being covered by an elastic cover.
In the machine calender, the hard steel rolls apply pressure at the nip along a narrow line, and a relatively high pressure is applied where the density of the paper is high. As a result, an undesirable change in the density of the paper occurs, which may be detrimental to the uniformity of printing on the paper. The super calender solves the shortcomings of the machine calender to some extent, since the width of the nip is broadened due to the effect of the elastic cover. However, heat, which accumulates between the elastic cover and the roll, is detrimental to the durability of the cover, and, as a result, the cover has a tendency to flake off the roll.
Recently, a calender apparatus using an endless belt comprising an elastic material was proposed to solve the problems of the machine calender and the super calender. Representative examples are shown in FIGS. 8 and 9.
In the calender apparatus shown in FIG. 8, a paper sheet W, which is placed on an elastic belt 1, is passed through the nip Pa formed between upper and lower steel rolls P1 and P2. The elastic belt 1 is an endless belt, which follows a path around roll P2, the path being relatively long compared to the circumference of roll P2. The upper roll P1 is heated by a heating apparatus (not shown). When the paper sheet W on the upper surface of the long elastic belt reaches the nip Pa and is sandwiched by the upper and lower rolls P1 and P2, its first surface W1, which is in contact with the press roll P1, is made smooth, but the second surface W2, which is in contact with the long elastic belt 1, is not made as smooth as the first surface W1, due to the effect of the surface of the belt. The density of the paper sheet W will not change greatly, and the paper sheet will have a surface suitable for printing. If a high smoothness is also necessary on the second surface W2 of the paper sheet W, it may be achieved by using another calender apparatus which does not use the elastic belt 1.
In a calender apparatus shown in FIG. 9, a paper sheet W, which is placed on a relatively short elastic belt 1, is passed through the nip part Pb formed between a steel roll P3 and a press shoe S. The short elastic belt 1 is an endless belt which travels around the press shoe P2 in a relatively short path. A lubricant is supplied to the inside surface of the belt 1 from time to time.
The calendered effect on the first surface W1, which contacts the steel roll P3 at the nip Pb, is no different from the effect achieved in the apparatus of FIG. 8. However, the smoothness of the second surface W2, which contacts with the elastic belt 1, may be superior to the smoothness of the corresponding surface of the paper calendered by the apparatus of FIG. 8, since the width of the pressurizing nip Pb may be larger where a press shoe is used. The calender apparatus shown in FIG. 9, in which the nip is formed by a press shoe, also has the advantage that it is easier in such an apparatus to prevent dispersion of oil supplied to the inside of the elastic belt. In a calender apparatus such as shown in FIG. 8, preventing dispersion of oil is more difficult.
Two characteristics, in particular, are demanded in an elastic belt used in both kinds of calender. One characteristic is flexibility of the high molecular weight elastic layer on the side which contacts the paper sheet. The other characteristic is durability of the part of the belt which is in contact with the press side. Proposals made in the past to meet these demands include, for instance, the proposal disclosed in unexamined PCT National Phase Publication No. 501852/1998 and the proposal disclosed in Japanese unexamined Patent Publication No. 88193/1985. Unexamined PCT National Phase Publication No. 501852/1998 discloses the elastic belt shown in FIG. 10, and Japanese unexamined Patent Publication No. 88193/1985 discloses of another elastic belt shown in FIG. 11.
The elastic belt 1′, shown in FIG. 10, has a base body 2 to impart strength to the belt as whole, a high molecular weight elastic layer 3 on the paper sheet side, which covers the paper sheet side 2a of the base body, and a high molecular weight elastic layer 4 on the press side, which covers the press side 2b of the base body opposite to the side 2a, the press side being the side facing a press roll or press shoe. The base body 2 is composed of a warp and a weft. In addition, to meet the above-mentioned demands, the high molecular weight elastic layer 3 of the paper sheet side is made flexible, and the high molecular weight elastic layer 4 of the press side is formed with a hardness higher than that of the high molecular weight elastic layer 3 on the paper sheet side. Thus, the layer 3 on the paper sheet side of the belt 1 is capable of adapting to the ruggedness of the paper sheet flexibly, and the press side layer 4 contributes to improved durability.
The elastic belt 1″, shown in FIG. 11, has a high molecular weight elastic layer 3′ which covers the paper sheet side 1a of a base body 2. The base body 2, which comprises a woven fabric having a warp and weft, is exposed on the press side 1b. The base body 2 imparts strength to the elastic belt 1″. The high molecular weight elastic layer 3′ forming the paper sheet side, has dispersed bubbles 5, and is produced by spreading a resin material on the base body 2 by spraying.
In the case of the conventional elastic belt 1′ shown in FIG. 10, the flexible cushion properties are brought into full play only by the properties of the resin of the high molecular weight elastic layer 3 on the paper sheet side. The structural strength of the belt is likely to become insufficient, and there is a possibility that elongation and breakage will occur. There is also the possibility that the elastic layer 3 on the paper sheet side will peel off the base body 2.
On the other hand, although flexibility may be achieved by the bubbles contained in the layer 3′ in the elastic belt 1″ shown in FIG. 11, this belt also has drawbacks. The manufacturing process is time-consuming, since the bubbles are produced by a spray jet. There is also the problem that the bubbles which are generated in the liquid plastic material are subject to shrinkage and are not stable in size.
An object of this invention is to solve the various problems of conventional elastic belts discussed above, and to provide an elastic belt which has superb flexibility and cushioning properties, making it especially suitable for use in a papermaking calender.